Sphingolipids are complex lipids containing as their backbone sphingosine or a related base. These important membrane components in both plant and animal cells are now believed to perform critical regulatory roles in cell growth and differentiation. Functioning as "lipid second messengers,"these molecules are part of a cascade of biochemical events which transmit signals from a broad array of circulating external stimuli, eventually triggering events within the cell nucleus. These external stimuli, considered "first messengers,"include growth factors, hormones, and neurotransmitters. This signal transduction cascade also involves modulating the activity of certain key intracellular enzymes. Protein kinase C (PKC) is an example of a group of such regulatory enzymes.
PKC has evolved as a major target for therapeutic intervention in diseases involving disordered cell growth and inflammation. Certain sphingolipid derivatives are now known to be potent inhibitors of PKC activity and are therefore considered to be candidates for novel therapeutic agents. One major clinical application now being evaluated is overcoming multi-drug resistance in certain cancers by inhibiting ("down-regulating") an intracellular, PKC-modulated "drug pump."During development and testing of certain sphingolipids for use as potential drugs, the need arose for a pharmaceutically acceptable vehicle to allow for a safe intravenous administration into animals and eventually into humans. Sphingolipids contain a molecule of sphingosine or one of its derivatives or isomers. One end of the sphingolipid, referred to as the tail, possesses a lipophilic, long-chain fatty acid. The other end, referred to as the head, is a hydrophilic group (see FIG. 1). Sphingosine, possessing a primary amine group, is cationic (basic) at physiological pH (pK.sub.a .about.6.5). With a non-polar lipophilic tail and a polar hydrophilic head, this molecule is considered to be amphiphilic and surface-active since it tends to orient at the interface between oil and water phases.
Sphingolipids are poorly water-soluble and tend to form large aggregates when placed in an aqueous environment by themselves. Therefore, it is desirable to be able to employ sphingolipids in a lipid-based delivery system such as an oil-in-water emulsion. However, attempts by the inventor of the present application to incorporate sphingolipids into conventional soybean oil emulsions, stabilized by mixed phospholipids purified from egg yolk, were unsuccessful. This is quite unfortunate since phospholipids purified from soybeans or egg yolk are the stabilizers of choice for oil-in-water emulsions intended for intravenous administration, including those to be used for parenteral nutrition as well as for those designed lipid-based drug delivery. Typically, the dispersed oil droplets in these emulsions are stabilized by layers of associated water molecules as well as by a net negative surface charge. In fact, the major component in phospholipid mixtures, phosphatidyl choline, is zwitterionic in form and neutral over a wide pH range. Therefore, this surface charge, also known as the Zeta potential, actually derives from small amounts of naturally occurring ionized lipids in the phospholipid mixture as well as from trace levels of free fatty acids which are hydrolyzed from the emulsifier and/or the oil phase during processing and sterilization at elevated temperatures.
Phospholipid-stabilized fat emulsions of the type described above are well-suited for parenteral nutrition or for the intravenous delivery of non-ionic lipophilic drugs. Well-known examples of the former include Intralipid.RTM. and Liposyn.RTM.. Good examples of the latter include Dizac.TM., which contains the sedative diazepam, and Diprivan.TM., which delivers the anesthetic propofol. In cases where the pharmacological agent contains an anionic functional group at physiological pH, emulsion droplet Zeta potential may become even more electronegative, often resulting in enhanced physical stability. However, applicant tried unsuccessfully many times to produce a phospholipid-stabilized fat emulsion containing a sphingolipid.